Determining Molecular Orientation With Surface-Enhanced Raman Scattering Using Inhomogenous Electric Fields

The inhomogenous electric field near the metal surface of plasmonic nanoparticles allows molecular orientation to be determined from surface-enhanced Raman scattering (SERS). We illustrate this by simulating the effects of the field-gradient on the SERS spectrum of benzene and pyridine. To do this, we present an origin-independent formalism describing the effects of the local electric-field gradient in SERS. Using this formalism, we found that the field-gradient led to observation of Raman-inactive modes in benzene and allowed for extraction of orientation information from the SERS spectra of both benzene and pyridine. It was also observed that the SERS electromagnetic enhancement factor, when considering field-gradient effects, depends on the field-gradient magnitudes and is only approximately described by |E|4 for certain modes. The field-gradient mechanism may also lead to a weakening of intensities as compared to a homogeneous local field. Thus, inclusion of field-gradient effects are crucial in understanding relative intensity changes in SERS.